1. Field of the Invention
The present invention relates to a process for producing ultra-pure water, an apparatus for producing ultra-pure water and a process for using the ultra-pure water produced according to the process. In particular, the invention relates to a technique for producing ultra-pure water from a raw water containing volatile substances such as carbonic acid component, volatile organic substances and the like and nonvolatile substances such as fine particles, microorganisms and the like. Further, the invention also relates to processes for using the ultra-pure water produced according to the process, such as the process for washing and drying a substrate with ultra-pure water in the manufacture of semiconductor wafer, optical disk and the like, the process for oxidizing the silicon surface of semiconductor wafer with ultra-pure water, and the like.
2. Prior Art
In the manufacture of semiconductors and medical drugs, a water of high purity containing as small a quantity of impurities as possible, i.e. ultra-pure water, is required. Particularly, a large quantity of ultra-pure water is used in the washing process of semiconductor integrated circuit (LSI). The purity of ultra-pure water exercises an important influence upon the yield of product. In the currently developed high level LSI's of 1 megabit, 4 megabits, etc., the washing process requires a more enhanced purity of water.
The prior apparatus for producing ultra-pure water is constructed of various filter membranes, ion exchange towers, a germicidal lamp, a deaerating apparatus, etc. as mentioned in Kankyo Gijutsu, Vol. 14, No. 4 (1985), pp. 353-358. FIG. 12 illustrates the outlined view of prior apparatus for producing ultra-pure water. The raw water is subjected to pre-treatments such as aggregation and precipitation, filtration, microfiltration, etc., after which it is sent to a step of reverse osmosis (RO) where the major part of soluble organic components and 90 to 99% of the inorganic salts present in the raw water are removed. The water having permeated the RO membrane is then decarbonated in the deaerating tower, after which it is sent to the ion exchange resin step. The ion exchange resin tower is of two bed/mixed bed regeneration type. In this step, salts are completely removed, and usually a primary pure water having a specific resistance of 10 M.OMEGA..multidot.cm or above is obtained in this step and stored in a pure water tank. The pure water is further treated with mixed bed type polishing ion exchange resin to remove impurities more completely. Then, it is made free from microorganisms in the ultra-violet germicidal step and then made free from the residual fine particles and dead microorganisms in the ultrafiltration (UF) step to give an ultra-pure water.
Apart from above, a pure-water producing apparatus using a hydrophobic porous membrane has also been proposed as mentioned in Japanese Patent Application Kokai (Laid-Open) No. 230,703/86. In this apparatus, the inner space is partitioned by a hydrophobic porous membrane which is permeable to vapor of liquids and impermeable to liquids themselves, and one side is used as the path of raw water, while the other side is provided with a cooling plate. The space between the cooling plate and the hydrophobic porous membrane is used as a condensing chamber, and outside of the cooling plate is used as the path of cooling water. The apparatus at least has a separating membrane unit for condensing the vapor supplied from the raw water path and having passed the porous membrane in the condensing chamber and a heat tank having a heater. First, a raw water (e.g. service water or the like) is led to the cooling water path of the separating membrane unit, and the raw water having passed the cooling water path is further led to the heat tank, and the raw water having been heated in the heat tank to a high temperature is led to the raw water path of the separating membrane unit, by pipings.
The above-mentioned prior art involve many elementary instruments such as filter membranes, ion exchange resin, germicidal lamp and the like, and improvement in the water quality of the resulting ultra-pure water necessarily requires to improve the quality of the elementary instruments. In the existing membrane module systems, there are a number of requirements as follows. Thus, the surface of the membrane having a separating performance must be free from defect, and the water having permeated the membrane must be perfectly sealed from raw water. Further, the module must be so constructed that it involves no part allowing the residence of liquid and all the parts of module must be thoroughly washable to prevent the growth of bacteria. Further, the quantity of substances dissolving out of the module must be as small as possible. Thus, a special grade of membrane module is used for production of ultra-pure water. However, a periodic washing by flushing and chemical sterilization are necessary to the prevention of bacterial growth in module. Further, in the desalting process using ion exchange resin, contamination by external impurities must be prevented by some means at the time of regeneration. Further, as the polishing ion exchange resin of the final stage, non-regeneration type of high purity resin which has been thoroughly washed previously must be used.
As above, the prior techniques for producing ultra-pure water have been disadvantageous in that they require considerably many maintenance works for keeping the water quality and the many elementary instruments used therein cause dissolution of contaminative substances to lower water quality, and further the existence of residence part causes a deterioration of water quality.
Further in the prior apparatuses for producing pure water using a hydrophobic membrane, there have been provided no particular decarbonating apparatus nor de-"TOC" (Total Organic Carbon) apparatus for removing carbon dioxide and organic substances from raw water in the fore stage, so that they have been unable to give a high water quality enough for use in the washing of LSI.